I'm trying to mine mine btc AND namecoin on slush's server, but it's not working. I downloaded namecoind and put my payout address on slush's website, but it's been a week now and I haven't mined any namecoin. Do I have to use some special settings in guiminer?

Slush pool only mines namecoins on the getwork interface. Those coins are then paid to all pool users, stratum included. The hash rate of getwork only miners is low making finding a namecoin block hard so payouts are not so often as pools where their entire hashrate is mining namecoins.